In the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) Architecture, downlink and uplink transmissions are organized into one of two duplex modes. These modes are frequency division duplex mode and a time division duplex mode. Frequency division duplex mode uses paired spectrum to separate the uplink and downlink transmissions while the TDD mode uses a common spectrum and relies on time multiplexing to separate uplink and downlink transmissions.
With FDD, the acknowledgement for a transmission typically occurs a set number of subframes after the transmission has been received. For example, in many systems the acknowledgement is sent back to the network from the user equipment (UE) four subframes after receipt of the transmission. In TDD, depending on the TDD mode, the HARQ feedback is sent in a predefined manner to the network once a transmission is received.
In order to increase data throughput, carrier aggregation may be utilized in LTE-advanced systems. To support 3GPP carrier aggregation, a LTE-advanced UE may simultaneously receive or transmit on one of multiple component carriers. In some cases, component carriers utilize the same duplex mode, and the HARQ operation and scheduling of the component carriers is therefore relatively straightforward. However, in some cases a secondary component carrier may be operating in a different duplex mode than a primary component carrier. In this case, the HARQ operation and scheduling are currently undefined.